Re: [BUG] double fget() in vhost/net (was Re: [PATCH] vfs: move fdput() to right place in ksys_sync_file_range())

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On Sun, May 15, 2022 at 03:30:23AM +0000, Al Viro wrote:
> [tun/tap and vhost folks Cc'd]
> 
> here's another piece of code assuming that repeated fget() will yield the
> same opened file: in vhost_net_set_backend() we have
> 
>         sock = get_socket(fd);
>         if (IS_ERR(sock)) {
>                 r = PTR_ERR(sock);
>                 goto err_vq;
>         }
> 
>         /* start polling new socket */
>         oldsock = vhost_vq_get_backend(vq);
>         if (sock != oldsock) {
> ...
>                 vhost_vq_set_backend(vq, sock);
> ...
>                 if (index == VHOST_NET_VQ_RX)
>                         nvq->rx_ring = get_tap_ptr_ring(fd);
> 
> with
> static struct socket *get_socket(int fd)
> {
>         struct socket *sock;
> 
>         /* special case to disable backend */
>         if (fd == -1)
>                 return NULL;
>         sock = get_raw_socket(fd);
>         if (!IS_ERR(sock))
>                 return sock;
>         sock = get_tap_socket(fd);
>         if (!IS_ERR(sock))
>                 return sock;
>         return ERR_PTR(-ENOTSOCK);
> }
> and
> static struct ptr_ring *get_tap_ptr_ring(int fd)
> {
>         struct ptr_ring *ring;
>         struct file *file = fget(fd);
> 
>         if (!file)
>                 return NULL;
>         ring = tun_get_tx_ring(file);
>         if (!IS_ERR(ring))
>                 goto out;
>         ring = tap_get_ptr_ring(file);
>         if (!IS_ERR(ring))
>                 goto out;
>         ring = NULL;
> out:
>         fput(file);
>         return ring;
> }
> 
> Again, there is no promise that fd will resolve to the same thing for
> lookups in get_socket() and in get_tap_ptr_ring().  I'm not familiar
> enough with the guts of drivers/vhost to tell how easy it is to turn
> into attack, but it looks like trouble.  If nothing else, the pointer
> returned by tun_get_tx_ring() is not guaranteed to be pinned down by
> anything - the reference to sock will _usually_ suffice, but that
> doesn't help any if we get a different socket on that second fget().
> 
> One possible way to fix it would be the patch below; objections?
> 
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

Suspect you are right, didn't test yet. Jason?

> ---
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 792ab5f23647..86ea7695241e 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -1450,13 +1450,9 @@ static struct socket *get_raw_socket(int fd)
>  	return ERR_PTR(r);
>  }
>  
> -static struct ptr_ring *get_tap_ptr_ring(int fd)
> +static struct ptr_ring *get_tap_ptr_ring(struct file *file)
>  {
>  	struct ptr_ring *ring;
> -	struct file *file = fget(fd);
> -
> -	if (!file)
> -		return NULL;
>  	ring = tun_get_tx_ring(file);
>  	if (!IS_ERR(ring))
>  		goto out;
> @@ -1465,7 +1461,6 @@ static struct ptr_ring *get_tap_ptr_ring(int fd)
>  		goto out;
>  	ring = NULL;
>  out:
> -	fput(file);
>  	return ring;
>  }
>  
> @@ -1553,7 +1548,7 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>  		if (r)
>  			goto err_used;
>  		if (index == VHOST_NET_VQ_RX)
> -			nvq->rx_ring = get_tap_ptr_ring(fd);
> +			nvq->rx_ring = get_tap_ptr_ring(sock->file);
>  
>  		oldubufs = nvq->ubufs;
>  		nvq->ubufs = ubufs;




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